Preparation of barretters



Nov. 17, 1964 H. L. MARTIN PREPARATION OF BARRETTERS Filed May 17, 19622 Sheets-Sheet l INVENTOR.

HOWARD L. MART/N QJAMIIW 7' A TTORNEY Nov. 17, 1964 H. L. MARTINPREPARATION OF BARRETTERS 2 Sheets-Sheet 2 Filed May 17, 1962 OSCILLATORETCHING CONTROL CIRCUIT INVENTOR. Haw/m0 L. MART/N BY QJMAZIM A 77 ORNEY United States Patent 3,157,5s7 PREhARATlUN 0i BARRET'EERS Howard L.Martin, Safety Harbor, Fla, assignor to Sperry Rand Corporation, Greatech, N.Y., a corporation of Delaware Filed May 17, 1962, Ser. No. 1%,5398 Qiairns. C1. 254-228) This invention relates to an improved method andapparatus for producing barretters of the type employed for powermeasurements at microwave frequencies.

Barretters of the type with which this invention is concerned areextremely thin wires of such small size that they are hardly visible tothe naked eye. They have the electrical characteristics that theirresistance changes with temperature and they are utilized by placingthem in a microwave transmission line such as a hollow waveguide or acoaxial transmission line, and by connecting them as one element of asuitable bridge circuit. Electromagnetic energy absorbed by thebarretter causes its resistance to change, thus changing the conditionof balance or unbalance of the bridge circuit. The change in resistanceof the barretter can be used to determine the amount of electromagneticenergy absorbed by the barretter by any one of several differentmethods. Detailed descriptions of barretters and their operation areoutlined in sections 3.23 and 3.24 of Techniques of MicrowaveMeasurements, by Montgomery, McGraw-Hill Book Company, Inc., New York,New York, copyright 1947.

The power sensing element of the more commonly used sensitive barrettersconsists of a short length of extremely fine platinum wire. The diameterof the platinum wire is in the order of 30 to 60 microinches. ince wireof this size is impossible to draw directly and would be very difiicultto handle it it were, it is necessary to use the Wollaston process toobtain Wire of this extremely small size. Wollaston process platinumwire is made by drawing the platinum inside a silver jacket or coating.By properly proportioning the diameters of the platinum rod and silverjacket at the start of the forming and drawing process, the finaldiameter of the platinum core can be controlled to rather close limits.The silver jacket in addition to providing a means for drawing theplatinum to such small diameters also provides the additional strengthand body necessary to handle the wire during the initial manufacturingsteps, and also provides a con venient means for achieving the exactresistance value of the barretter wire. This resistance value isobtained by removing the silver jacket over a portion of the length ofthe Wollaston wire to leave exposed the higher resistance platinum core.Because of the very fine nature of the Wollaston wire it is difiicult towork with and breakage of the wire during the etching process to removethe silver jacket and during the assembly of the wire into a suitableholder can be quite high. Additionally, the desired control of theetching process may be ditficult. Because the present drawing processesdo not provide precise control of the Wollaston wire diameter, it is notpossible to etch the wire to a certain fixed physical length and beassured of obtaining the desired resistance value. Control of theetching process is important because the finely etched wire must havespecific electrical and physical characteristics. It is very importantthat all of the silver be removed from the platinum core throughout theetched area because any silver particles not removed from the platinumcore may alloy with the platinum at high temperatures and change theresistance value of the etched wire.

0ne method for producing barretters is disclosed in US. Patent2,468,796, assigned to the applicants assignee. This method has beenused successfully but sufiers from the disadvantage that it requireshighly skilled personnel,

and the useful output yield of that method is not always as high asdesirable because it is difiicult to obtain uniformity of etching on aproduction basis and because the silver particles cannot be completelyand uniformly removed from the etched portion of the platinum wire bythis method.

It therefore is an object of this invention to provide an improvedapparatus and method for producing barretters that have improved andconsistently repeatable electrical characteristics.

It is another object of this invention to provide an improved method formaking barretters that permits closer control of the etching processresulting in more uniform and consistent barretters.

It is a further object of this invention to provide a semiautomaticapparatus for etching barretter wires, wherein the semi-automaticfeature affords precise control over the etching process.

It is another object of this invention to provide means for accuratelycontrolling the area to be etched and the rate of etching of a Wollastonwire, which together with automatic control of the etching current,produces a greater yield of barretter wires of superior characteristics.

in accordance with the present invention, a Wollaston wire to beelectrolytically etched is supported on an assembly fixture betweenaxially aligned supporting members. The ends of the supporting membersand the ends of the Wollaston Wire are masked by enclosing them within asuitable material that is resistant to electrolytic etching. The maskingmaterial is applied so as to leave exposed a fixed central portion ofthe wire. A droplet of a suitable electrolyte is brought into contactwith the central unmasked portion of the wire so as to completelysurround the wire. The wire is connected as the unknown resistanceelement in an A.C. resistance measuring bridge circuit, and a DC.electrolytic etching current circuit is established between the wire andthe droplet of electrolyte. The etching current circuit includes anelectrically controllable current interrupting element that iscontrolled by the condition of balance or unbalance of the bridgecircuit. That is, the current interrupting means maintains the etchingcurrent circuit in a current conducting condition when the bridge isunbalanced and breaks the circuit when the bridge is in a balancedcondition. During the initial stage of etching of the Wollaston wire,the unmasked central portion of the silver coating is removed at arelatively rapid rate, but the masked portion of the wire has beenselected so that the desiredresistance value has not yet been obtained.The etching then proceeds at a slower rate to remove the ends of thesilver coating under the masking material, and to thoroughly clean upthe etched central portion of the wire. This etching continues at aslower rate until the desired resistance value is reached. When theWollaston Wire has been etched so as to achieve its desired resistancevalue, the bridge will be in a balanced condition and the currentinterrupting means becomes deenergized and opens the etching currentcircuit, thereby automatically terminating the etching process.

The present invention will be described by referring to the accompanyingdrawings wherein,

FIG. 1 is a perspective view of an assembly fixture on which a barretteris made, and is used in explaining the method of assembling thebarrette'rs;

PEG. 2 is a simplified sketch, partly in schematic form, illustratingthe electrical equipment and control apparatus, and the masking materialused in the electrolytic etching of the silver coating from a portion ofa Wollaston wire;

FIG. 3 is an illustrative sketch of the view of a partially assembledbarretter as seen through a microscope rial.

and is used in explaining the manner in which the masking material isapplied to the Wollaston wire; and

FIG. 4 is a simplified illustration of the etched resistance wire priorto encapsulation.

Referring now in detail'to FIGURE 1, the assembly fixture 19 that isused in assembling the barretters is cornposed of upstanding end postsHand 12, both supported on a base 13. End posts 11 and'iil areelectrically insulated from base 13, and from each other, and for thispurpose base 13 may be made of a non-conductive mate- Extendingtransversely through end members 11 and 12 are respectiveaxially-movable conductive positioning members 16 and 17, both of whichare slidable through transverse holes in the respective end members 11and 12. Positioning member 3.6 has an enlarged collar 18 at its left endfor fixing the position of member 16 with respect to end post 11.Positioning members 16 and 17 are bored'at their adjacent inner ends toreceive the end members 22 and Zito which are secured thepower-absorbing resistance wire 22, a Wollaston wire as previouslydescribed. End members 20 and 21 have respective closing plugs 24 and 25thereon which will form the ends of the finally'encapsulated barretter.The

- closing plugs 24 and 25 may be in the form of collars of greaterdiameter that are machined as integral parts of end members 2% and 21,or may be separate annular plugs that slip'over the end members 2% and21 and may be fastened to the end members some time during the assemblyprocess. may be secured against axial movement with respect to end posts11 and 12 by means of respective set screws 28 and 29.

The means for introducing a droplet of an electrolyte into contact withWollaston wire 22 is illustrated by the apparatus 30 which is comprisedof tool holder 31 to which is secured the tool 32 that has a smallreservoir Positioning members 16 and 17 33 at its end for supporting adroplet 34- of electrolyte,

which may be a ten percent solution of potassium cyanide. Tool holder 31is vertically movable by rotation about an axial pin 45 that is securedbetween vertical posts 46 and 47. Tool holder 31 may be clamped inposition by turning a handle 48 that turns screw 49 to bring posts 46and 47 tightly against tool holder 31. Tool holder 31 may be movedlongitudinally parallel to the wire 22 by means of enlarged holes 51 and52 through which pass the clamping screws 55 and 5s. The length of tool32 may be adjusted by suitable means not illustrated. As

'now may be understood, droplet 34 of the electrolyte is positionableboth vertically and longitudinally to bring it into contact with theportion of the resistance wire 22 to be electrolytically etched.

To accomplish the electrolytic etching of the silver cordance with thisinvention, a direct current electrolytic etching current is passedthrough the reservoir 33, through droplet 34 of the electrolyte, toWollaston wire 22 in such a direction as to remove or deplate the silverjacket from the Wollaston wire, thus exposing the platinum core of thewire. The electrical circuitry for accomplishing this electrolyticetching in accordance with this invention is illustrated in simplifiedschematic form in FIG. .2. The positioning members 16 and 17 with theend members 20, 21 and Wollaston wire 22 conductively connectedtherebetween are connected as one leg of an A.C.'bridge circuit 81,which may be of any of the types commonly used in the art. An oscillator82 is connected across the bridge circuit 81, and an AC. am plifier' 60is connected to one point of the bridge in a manner to be energized bythe bridge circuit when it is in its unbalanced condition. Substantiallyno input signal flows to amplifier 66 when the bridge is balanced. Theoutput of amplifier 60 is coupled to etching control circuit 83 whichwhen energized by the output of amplifier 60 maintains a closed etchingcurrent conduction path from a DC. source of etching current E throughposi- 7 jacket or coatingrfrom the Wollaston wire 22 in actioning member17, end member 21, Wollaston wire 22,

reservoir 33, tool holder 31, and through the etching control circuit 83back to the other terminal of source E Etching control circuit 83 may becomprised of a relay actuated switch, for example. When bridge 81'is' inthe balanced condition, there is no output from amplifier 60 and etchingcontrol circuit 83 operates to open the electrolytic etching currentpath, thus terminating the etching process. The use of an A.C.resistance measuing bridge circuit 81 and A.C. amplifier 69 providesisolation from the DC. etching current circuit since amplifier 6% willblock DC. signals, thus making the operation of. etching control circuit83 independent of theetching current flowing and rendering its operationdependent entirely upon the resistance of the etched Wollaston wire 22as monitored by bridge circuit 81.

The method of producing barretters in accordance with the presentinvention is carried out in the following manner. Referring to FIG. 1,end members 26 and 21 are positioned in the respective ends ofpositioning members 16 and 17, and positioning member 16 is moved.

inwardly until collar 13 is in intimate contact with end post 11. Setscrew 23 then is turned in to secure positioning member 16. Positioningmember 17 is positioned a fixed axial distance from the end of member 16by means of some suitable gage tool and then secured in position by'setscrew 2?. Set screw 23 is backed off, positioning means 16 partiallywithdrawn, and a thin shim 620i the order of .005 inch thick is placedbetween collar 13 and end post 11 for reasons that will become apparentfrom the later description. Positioning member 16 then is moved inwardlyto its limit fixed by collar 18 and shim 62, and is secured by setscrew- 28. An encapsulating sleeve 65, which may be polystyrene or someother suitable low loss dielectric mateial, then is slipped over the endof end member 21 and positioning member 16 and is moved to the left awayfrom the space between end members 20 and 21. Resistance wire 22, 'aWollaston wire, then is secured to the adjacent ends of end members 20and 21 by solder ing. Resistance wire 22 may be soldered into smallholes on the ends of end members 20 and 21, or may be soldered to theouter surfaces of these members. I have found that resistance heating ofthe soldered joints by means of a pencil tip soldering member producesexcellent results, being sure to maintain the pencil tip in good contactthroughout the entire soldering period in order to prevent arcing andburning of' end members 29 and 21. Because end members 20 and 21 oftenare proportioned to have specific microwave properties in a transmissionline, it is important that they not be damaged in this or in subsequentsteps.

Reference now will be made to FIG. 3 which is intended to represent theview seen through the eyepiece of a miscroscope that is positioned overthe assembly fixture illustrated in FIG. 1. The eyepiece of a microscopeis provided with a reticle comprised of four spacedapart vertical lines71, 72, '73 and 7d. The microscope is adjusted so that ,theoutermostvertical lines 71 and 72 are coincident with the adjacent'end-s of pinmembers 26 and 21. A masking material that is resistant to theelectrolytic process then is applied to cover the ends of pin members 2%and 21 and also extends onto a portion of the outer ends of resistancewire 22 so as .to form the protective .cones 68 and 69. As a maskingmaterial wise would remove the gold or other plating from end member 2%and 21 and also would attack the exposed The length based metals of endmembers 24 and 21.

of resistance wire 22 that remains unmasked desirably is shorter thanthe length of the Wollaston wire that finally is etched. As will beexplained hereinafter, this feature allows for precise control of theetching and assures substantially complete removal of the silver coatingfrom the central portion of the platinum core of the Wollaston wire. Ifthe etching process were not confined by properly positioned maskingcones 68 and 69, the silver coating would be removed from the Wollastonwire in a more or less un form manner until the platinum core was almostcompletely exposed over its entire length, and this would result in anexcessively over-etched and excessively high resistance barretterelement.

In the next step of the assembly of the barretter, a droplet 34 of theel ctrolyte is placed Within reservoir 33, and by means of the apparatus31 of FIG. 1, is brought into contact with the unmasked central portionof resistance wire 22 so as to completely surround the unmasked portionin the manner illustrated in FIG. 2. The appropriate electricalconnections then are made in accordance with the schematic diagram ofFIG. 2 to insert the resistance wire 22 into one leg of the bridgecircuit 81, and to establish the electrolytic etching current path thatincludes a source of DC. potential E positioning member 17, end member21, resistance Wire 22, droplet 34 of the electrolyte, reservoir 33,tool 31, and plating control circuit 83. With bridge circuit tilproportioned to be in a balanced condition when resistance wire 22 hasbeen electrolytically etched to its desired resistance value, the bridgeinitially will be in an unbalanced condition since the resistance of theunetched Wollaston wire is relatively low. In this unbalanced conditionof bridge circuit 81, amplifier oil will be energized by an unbalancedA.C. signal from bridge circuit 81 and will provide a signal to etchingcontrol circuit 83 which in turn operates to close the electrolyticetching current conducton path, thus permitting the electrolytic etchingprocess to commence. The electrolytic etching process will proceedrelatively rapidly until the silver coating is substantially removedfrom the unmasked central portion of Wollaston wire 22. In thiscondition, the resistance of Wollaston wire 22 still is lower than itsdesired final value so that bridge circuit 81 s ill is in an unbalancedcondition and the electrolytic etching current path still is maintainedclosed by the etching control circuit 83. The electrolytic etchingprocess now proceeds at a slower rate by removal of the ends of thesilver coating that lie under the protective cones 68 and 69. Duringthis period of time the electrolytic etching continues on the exposedplatinum wire so as to thoroughly clean up the last vistage of silverthat remains after the early and rapid removal f the silver coating fromthe unmasked portion of Wollaston wire 22. It may be seen that thisfeature of the etching process will permit substantially completeremoval of all silver particles from the erposed portion of Wollastonwire 22 and results in a superior etched wire Whose characteristics willnot substantially change with use. The etching process then proceedsslowly on the ends of the silver coating under the masking cones 68 and69 until the desired resistance value is obtained. When this resistancevalue is achieved, bridge circuit 31 will be in a balanced condition andthe output of amplifier 69 will be substantially zero. In the absence ofan input to etching circuit 83 from amplifier so, etching controlcircuit 83 o crates to open the electrolytic plating current path andthus terminate the electrolytic etching process. it thus may be seenthat the electrical control circuitry illustrated in FIG. 2 providesmeans for stopping the electrolytic etching process at the precisemoment that the desired value of resistance has been obtained, andinasmuch as a chemical etching solution is not employed, no furtheretching action will take place. It may be seen that the manufacture ofbarretters by the method of this invention may be carried out byrelatively unskilled personnel inasmuch as the automatic control of theetching process completely eliminates the requirement that the operatorrely upon his judgment to determine when the etching process should beterminated. If desired, an indicating light may be added to etchingcontrol circuit 83 to indicate when the etching process has beencompleted. Further, the use of the electrolytic etching process and theprotective cones 68 and 69 produces an etched wire or" superior qualityhaving substantially no particles of silver remaining on the etchedportion, and these superior results are constantly repeatable due to theautomatic control of the etching process. A further advantageous featureor" this invention is that an operator may attend more than one assemblyfixture at the same time since his attention is not required once theetching process has commenced.

At the conclusion of the etching or deplating process just described,the droplet of electrolyte 34 is Withdrawn from contact with the etchedwire 22. Because of the very delicate nature of the etched Wire, extremecare must be used in order that the surface tension of the droplet 34with the Wire does not cause the wire'to break as the droplet isWithdrawn. In order to reduce the surface tension, a suitable wettingagent may be mixed with the electrolyte to reduce its surface tension.The operator then proceeds with the assembly of the barretter bycleaning the etched resistance wire to remove the electrolyte, any loosemetallic particles, and the protective cones 68 and 69. This may beaccomplished by inserting a portion of the assembly fixture of FIG. 1into an ultrasonic cleaning tank, and then rinsing with grain alcohol.Following the cleansing operation, set screw 28 is partially backed offand shim 62 is withdrawn from the fixture. Conductive positioning member16 then is carefully pushed inwardly until shoulder 18 abuts against theouter surface of end member 11. Set screw 28 then is tightened to securepositioning member 16. This causes a slight bend to form in the etchedportion of the Wollaston wire 22, see FIG. 4, thereby relieving anytension on the wire and rendering it less susceptible to breakage due toshock and vibration. The resistance of the etched wire 22 again may bechecked to make sure that it meets the desired requirements and, ifacceptable, the encapsulating sleeve 55 of FIG. 1 is moved to the rightso that its two ends engage the respective closing plug portions 24 and25 on end members 20 and 21. Encapsulating sleeve 65 then is secured toclosing plugs 24 and 25 by any suitable means such as an adhesive.

While the invention has been described in its preferred embodiments itis to be understood that the Words which have been used are words ofdescription rather than of limitation and that changes Within thepurview of the appended claims may be made without departing from thetrue scope and spirit of the invention in its broader aspects.

What is claimed is:

1. Apparatus for use in electrolytically etching a power absorbingresistance wire comprising,

means "for supporting two conductive members in spaced apartrelationship,

means for supporting an electrolyte in a region between said conductivemembers, means connected to said spaced conductive members comprisingthe arms of known resistance values of a resistance measuring bridgecircuit,

means for connecting a source of direct current between one of saidspaced conductors and said electrolyte supporting means, and

means responsive to an output signal of said bridge circuit todisconnect said direct current source only when said output signal is ofa predetermined value.

2. Apparatus for use in electrolytically etching a power absorbingresistance wiring comprising,

means for supporting two conductive members in spaced apartrelationship,

said conductive members being adapted to have a length of powerabsorbing resistance wire extended therebetween, 7

an electrode disposed in a region between said conductive members,

circuit means connected between said conductive members comprising thearms of known resistance values of a resistance measuring bridgecircuit,

said conductive members comprising terminals of the arm of unknownresistance value of said bridge circuit,

a source of direct current etching current connected between one of saidspaced conductors and said electrode, and

means operable in response to a condition of balance of said bridgecircuit to disconnect said direct current source when said bridgecircuit is in a condition of balance.

3. The combination claimed in claim 2 and further including,

an alternating current signal source for energizing said bridge circuit.

4. The combination claimed in claim 3 and further including directcurrent isolating means coupled between said bridge circuit and saiddirect current disconnect means.

-5. Apparatus for use in electrolytically etching a power absorbingresistance wire comprising,

means for supportingtwo conductive members in spaced apart relationship,

an electrode disposed between said spaced conductive members for use inmaking electrical Contact with an electrolyte to beused inelectrolytically etching said wire,

means connected to said spaced conductive members comprising the arms ofknown resistance Values of a resistance measuring bridge circuit,

said conductive members comprising terminals of the arm of unknownresistance value of said bridge circuit, I

means for connecting a source of direct current etching current betweensaid electrode and one of said conductive members, and

means operable in response to an output signal of said bridge circuit todisconnect said direct current source when said output signal is of apredetermined magnitude. i I

6. Apparatus for use in electrolytically etching a power absorbingresistance wire comprising,

means for supporting two spaced apart conductors,

means for supporting an electrolyte in a region between said conductors,

means connected to said spaced conductors comprising the arms of knownresistance values of a resistance measuring bridge circuit,

said conductors comprising terminals of the arm of unknown resistanceValue of said bridge circuit,

means for connecting a source of direct current be tween one of saidspaced conductors and said electrolyte supporting means, and

means operable in'response to a condition of balance of said bridgecircuit to disconnect said direct current source when said bridgecircuit is in a condition or" balance. 7. Apparatus for use inelectrolytically etching a power absorbing resistance wire comprising,

means for supporting two conductive members in spaced apartrelationship,

said conductive members being adapted to support therebetween a lengthof resistance Wire to be elec trolytically etched,

an electrode disposed between said spaced conductive members for use inmaking an electrical contact with an electrolyte to be used inelectrolytically etching said wire,

means for connecting a source ofdirect current etching current betweensaid electrode and one of said couductive members,

resistance measuring means connected to said spaced conductive membersfor use in determining the resistance value of a wire extended betweensaid conductive members,

an alternating current source coupled to said resistance measuringmeans,

whereby resistance measurements are made in response to alternatingcurrent signals that energize said re sistance measuring means,

direct current disconnect means included in said means for connecting asource of direct current between said electrode and one of saidconductive members,

said disconnect means operating to interrupt the direct current when theresistance measuring means productive members would comprise the known'resist.

ance arm of said bridge circuit, a source of direct current forsupplying an etching currentto etch a length of Wollaston wire supportedbetween said conductive members, means for coupling said direct currentsource to said reservoir and to one of said conductive members, andmeans operable in response to a condition of balance of said bridgecircuit for disconnecting said direct current source when said bridgecircuit is in a condition of balance.

References Qited in the file of this patent V UNlTED STATES PATENTS2,564,823 Vl/lallace Aug. 21, 1951,

1. APPARATUS FOR USE IN ELECTROLYTICALLY ETCHING A POWER ABSORBINGRESISTANCE WIRE COMPRISING, MEANS FOR SUPPORTING TWO CONDUCTIVE MEMBESIN SAPCED APART RELATIONSHIP, MEANS FOR SUPPORTING AN ELECTROLYTE IN AREGION BETWEEN SAID CONDUCTIVE MEMBERS, MEANS CONNECTED TO SAID SPACEDCONDUCTIVE MEMBERS COMPRISING THE AREA OF KNOWN RESISTANCE VALUES OF ARESISTANCE MEASURING BRIDGE CIRCUIT, MEANS FOR CONNECTING A SOURCE OFDIRECT CURRENT BETWEEN